In recent years, a number of marked advances have been made in the state of the art of inorganic solar cells. Notably, conversion efficiencies have been improved above the 5% level. Examples of such improved cells are given in U.S. Pat. Nos. 4,035,197 and 4,207,119, issued on July 12, 1977 and June 10, 1980, respectively.
However, before such cells can significantly replace more conventional power sources, a series-connected array of such cells is needed with significantly greater voltage outputs than an open-circuit voltage of 800 mV that can now be achieved from a single cell.
Although arrays of photovoltaic cells have been constructed, they have been beset with problems that have prevented significant commercialization. For example, arrays of series-connected solar cells have been constructed wherein the positive and negative electrodes of adjacent cells are mechanically joined together. Representative examples appear in U.S. Pat. No. 3,571,915, issued Mar. 23, 1971. Such cells have the disadvantage that mechanical connection of opposite electrodes results in wasted space between cells and less than the maximum packing density of cells per unit width of array. Furthermore, mechanical bonding of the electrodes is very time consuming and labor intensive, compared to step-to-step deposition processes that can be used for the remainder of the fabrication.
One approach to fabricating integrated arrays is to coat the active layers of the cells by a deposition from the vapor phase without using masks. Undesired portions are then removed. Examples of such an approach are disclosed in South African Pat. No. 78/3886. However, before the last electrode layer is applied, insulting beads are deposited along the exposed edges of the semiconductor strips, to prevent the last electrode layer from contacting the semiconductor portions of two adjacent cells, as would otherwise necessarily occur in an overall deposition coating that is, otherwise free of masks. One reason for this step is that both lower electrodes of two adjacent cells are exposed by the removal of the semiconductor material. Also, at least a portion of the semiconductor material, usually the p-type material in a heterojunction cell, has a conductivity that provides insufficient resistance to shorting in the event the outer electrode of two adjacent cells contacts the low-resistance material. However, the use of such beads of insulation represents a separate processing step requiring great care and precision, and accordingly produces a marked increase in the cost of the manufacturing process and materials.
Additional problems occurring with the simultaneous assembly of integrated arrays is the difficulty in detecting a short circuit in an individual cell prior to its assembly in the array. Once assembled, a short will show up as a power loss, but its exact location in the array is indeterminable, preventing it from being operatively removed from the array.
Therefore, there has been a need, prior to this invention, to reduce the cost of manufacturing integrated arrays, and at the same time to provide a technique for electrically isolating shorts when they occur in such arrays.